1. Field of the Invention
The present invention relates to techniques for producing ions for analysis of chemical compositions. More particularly, the present invention relates to improved techniques for ionizing electrospray droplets containing molecules of interest and solvent for mass spectrometric analysis of various chemical compounds.
2. Description of the Background
Mass spectrometry (MS) has long been a widely accepted analytical technique in the chemical sciences field to obtain qualitative and quantitative information from a sample. MS is commonly used to determine molecular weight, identify chemical structures, and accurately determine the composition of mixtures. MS is becoming increasingly important in biological research to determine the structure of organic molecules based on the fragmentation pattern of ions formed when sample molecules are ionized. According to MS, individual molecules may be "weighed" by ionizing the molecules and measuring the response of their trajectory to electric or magnetic fields. Improved techniques have been developed to analyze thermally unstable or nonvolatile molecules with high molecular weights over 10,000 Daltons.
Large molecules cannot be readily transformed into gas phase ions suitable for MS analysis using electron ionization, photo-ionization, and chemical ionization techniques due to the extensive decomposition of the molecules which occurs in these processes. Such extensive decomposition substantially reduces the accuracy of the analytical technique, and accordingly these ionization techniques are not generally preferred for analysis of large molecules. Intact ions from large molecules are increasingly being produced by "soft" ionization techniques, however, including field desorption, electrohydrodynamic ionization, thermospray, and electrospray. These soft ionization techniques do not significantly alter the molecules of interest, and are becoming increasingly used in conjunction with gas phase analyzers to determine the composition of samples. While each of these soft ionization techniques is preferred by certain users, electrospray is one of the more promising soft ionization techniques for the efficient production of ions for mass spectrometric analysis.
Electrospray ionization techniques were first proposed in the late 1960's. A sample solution containing molecules of interest and a solvent is pumped through a hypodermic needle and into an electrospray chamber. An electrical potential of several kilovolts may be applied to the needle for generating a fine spray of charged droplets. According to UK patent specification No. 1,246,709, the droplets may be sprayed at atmospheric pressure into a chamber containing a heated gas to vaporize the solvent. Alternatively, the needle may extend into an evacuated chamber, and the sprayed droplets then heated in the evacuated chamber by an infared filament. In either case, ions are focused into a beam, which is accelerated by an electric field gradient, and the ions then analyzed in a mass spectrometer.
Significant disadvantages are encountered if an electrospray is discharged into an evacuated chamber. The charged droplets are not retarded from migrating toward the chamber walls, thereby increasing the possibility of discharge and disruptions to the spray. U.S. Pat. No. 4,209,696 teaches an electrospray technique which occurs at atmospheric pressure or above, and the produced ions are input to a mass analyzer.
Although the electrospray jet is formed at atmospheric pressure, mass spectrometers or other gas phase detectors routinely operate within a vacuum chamber. A vacuum housing for the mass spectrometer typically includes a plurality of lenses in a vacuum chamber maintained at a low pressure of several Torr or less. The chamber is typically heated to about 100.degree. C. to keep the chamber and lenses clean. The gaseous components of the jet which are emitted into the mass spectrometer first pass through these lenses having orifices sized to maintain the desired pressure in view of the practical limitations of the vacuum pumps. A small percentage of the ions produced in the electrospray chamber pass through these orifices and into the analyzer, since most of the ions are removed with the solvent vapor by the vacuum pumps. Accordingly, the transfer efficiency (percentage of ions produced by the electrospray which actually enter the analyzer) is low, which substantially limits the sensitivity of the electrospray/MS technique.
Another significant problem with electrospray concerns the condensation of the expanding jet and clustering of the ions. To reduce this problem, heated counterflow gases are commonly employed to vaporize sprayed droplets and desolvate ions at atmospheric pressure. Since the heated counterflow gases remove much of the solvent vapor from the stream of gas before being drawn into the vacuum chamber, this technique increases the concentration of ions of interest in the vacuum chamber. U.S. Pat. No. 4,023,398 teaches a technique whereby ions pass through an orifice into a vacuum chamber, while a gas curtain upstream from the orifice reduces transmission of solvent vapor into the vacuum chamber. The gas is heated to hasten evaporation of the solvent from the droplets, thereby producing desolvated ions at substantially atmospheric pressure. U.S. Pat. No. 4,531,056 teaches a similar technique, whereby an inert gas is introduced into the electrospray chamber in a direction opposite to a flow from the capillary. The electrospray chamber remains at or slightly greater than atmospheric pressure. Ions of interest are produced within the electrospray chamber, and the inert gas flow substantially reduces the concentration of solvent vapor which enters the analyzer. U.S. Pat. Nos. 4,842,701 and 4,885, 076 disclose a system which combines capillary zone electrophoresis with electrospray for gas analysis of an analyte mixture. Again, the electrospray occurs at atmospheric pressure, and a heated countercurrent gas flow technique is used to desolvate the spray droplets.
While the counterflowing gas concept described above results in reasonable sensitivity, it substantially increases the complexity of the interface between the electrospray and the mass spectrometer. In order that the solvent vapor from the evaporating droplets be efficiently removed by the counterflowing gas, both the temperature and the flow rate of the gas must be carefully controlled. High gas flow rates may prevent some ions with low mobility from entering the analyzer, while low gas flow rates or reduced gas temperature may not sufficiently desolvate the ions. The counterflowing gas flow rate and temperature are typically optimized for each analyte and solvent. Accordingly, much trial and error time is necessary to determine the optimum gas flow rate and temperature for each particular analyte utilizing a particular electrospray device and a particular gas analyzer.
The disadvantages of the prior art are overcome by the present invention, and improved techniques are hereinafter disclosed utilizing electrospray techniques for improved ionization of molecules of interest to enhance the reliability and sensitivity of the mass spectrometric or other gas-phase analysis.